The Distribution and Trends of Fog and Haze in the North China Plain Over the Past 30 Years

Home , Taihang Mountains

Atmos. Chem. Phys., 14, 11949–11958, 2014 www.atmos-chem-phys.net/14/11949/2014/ doi:10.5194/acp-14-11949-2014 © Author(s) 2014. CC Attribution 3.0 License.

The distribution and trends of fog and haze in the North China Plain over the past 30 years

G. Q. Fu1,3, W. Y. Xu2,4, R. F. Yang1, J. B. Li5, and C. S. Zhao2 1Hebei Province Meteorological Service Centre, Shijiazhuang, Hebei, China 2Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China 3Key Laboratory for Meteorology and Ecological Environment of Hebei Province, Shijiazhuang, Hebei, China 4Key Laboratory for Atmospheric Chemistry, Institute of Atmospheric Composition, Chinese Academy of Meteorological Sciences, Beijing, China 5Hebei Province Meteorological Observatory, Shijiazhuang, Hebei, China

Correspondence to: C. S. Zhao ([email protected])

Received: 29 April 2014 – Published in Atmos. Chem. Phys. Discuss.: 19 June 2014 Revised: 29 September 2014 – Accepted: 30 September 2014 – Published: 13 November 2014

Abstract. Frequent low visibility, haze and fog events were ciated with haze days are evidently increasing, suggesting found in the North China Plain (NCP). Data throughout the that an increasing fraction of haze events are caused by the NCP during the past 30 years were examined to determine hygroscopic growth of aerosols, rather than simply by high the horizontal distribution and decadal trends of low visibil- aerosol loadings. ity, haze and fog events. The impact of meteorological factors such as wind and relative humidity (RH) on those events was investigated. Results reveal distinct distributions of haze and fog days, due to their different formation mechanisms. Low visibility, haze and fog days all display increasing trends of 1 Introduction before 1995, a steady stage during the period 1995–2003 and a drastically drop thereafter. All three events occurred most Low-visibility events caused by fog and severe haze events frequently during the heating season. Benefiting from emis- can be a heavy burden for air transport and on-road traffic. sion control measures, haze and fog both show decreasing The severe aerosol pollution that has led to the visibility im- trends in winter during the past 3 decades, while summertime pairment is also highly damaging to human health. haze displays continuous increasing trends. The distribution In developed countries such as the US and the Euro- of wind speed and wind direction as well as the topography pean countries, the employment of emission control mea- within the NCP has determinative impacts on the distribu- sures for sulfur, nitrogen, hydrocarbon compounds and par- tion of haze and fog. Weakened south-easterly winds in the ticulate matter has already resulted in significant declines in southern part of the NCP have resulted in high pollutant con- haziness and major improvements in visibility during the past centrations and frequent haze events along the foot of the 50 years (Schichtel et al., 2001; Doyle and Dorling, 2002; Taihang Mountains. The orographically generated boundary Molnár et al., 2008; Vautard et al., 2009). As a rapidly de- layer wind convergence line in the central area of the south- veloping country, however, air pollution problems have been ern NCP is responsible for the frequent fog events in this haunting China for the past few decades. Visibility has been region. Wind speed has been decreasing throughout the en- deteriorating in southeast (Deng et al., 2012) and southwest tire southern NCP, resulting in more stable atmospheric con- China (Fu and Wu, 2011) during the past 50 years, with only ditions and weaker dispersion abilities, calling for harder ef- a few places revealing slight increases in recent years. Six forts to control emissions to prevent haze events. Haze events Chinese megacities have suffered from continuously decreas- are strongly influenced by the ambient RH. RH values asso- ing visibilities in the past 30 years (Chang et al., 2009). Over- all, visibilities on clear days decreased significantly during

Published by Copernicus Publications on behalf of the European Geosciences Union. 11950 G. Q. Fu et al.: The distribution and trends of fog and haze in the North China Plain the period 1960–1990 over all of China; trends thereafter, however, have not been consistent (Wu et al., 2012). The visibility trends were often attributed to the decrease in sulfur dioxide emissions or aerosol concentrations; however, no causality could be established so far. Both SO2 and aerosol pollution have been proved most severe in the North China Plain (NCP), due to the rapid economic growth and the high population density (Xu et al., 2011). As a result, low-visibility events frequently occur. Many in situ measure- ments and studies have already been performed to study the light scattering properties of aerosols and their impact on horizontal visibility under different relative humidity (RH) in the NCP as part of the Haze in China (HaChi) Campaign (Chen et al., 2012; Ma et al., 2011). Results show that, under low RH, low-visibility events are mostly induced by the heavy aerosol loading, while under high RH, the influence of aerosol hygroscopic growth becomes stronger and can lead to low-visibility events even under moderate aerosol pollution levels. However, only few studies have been performed to in- vestigate the spatial distribution of low-visibility events and Figure 1. The location of the measurement sites (circle), the area their overall trends during the past few decades in the NCP. of study (red line) and the regional topography by Danielson and Zhao et al. (2011) analysed the visibility trends in the NCP, Gesch (2011) (colour). suggesting declining visibilities before 1998 and increasing ones during the period 2006–2008, with stronger visibility deterioration trends in the summer season due to higher RH and lower wind speeds. Both haze and fog events could thus were used to analyse the long-term temporal and spatial vari- be responsible for low-visibility events; however, those were ation of low visibility, haze and fog events. rarely differentiated in the study of visibility trends, which Both haze and fog can lead to low-visibility events. The made it difficult to find the reason behind those visibility vari- formations of haze and fog are two distinctly different pro- ations. cesses. Therefore, it is necessary to differentiate between In this study, a detailed analysis on the decadal variation those two events when analysing low-visibility trends, dis- and spatial distribution of low visibility, fog and haze events tributions and variations. In this work, low-visibility events in the most polluted southern part of the NCP will be per- were defined as days with visibility at 14:00 LT below 10 km. formed, and the impact of wind and RH on those events will Those low-visibility events that were not associated with fog, be revealed. precipitation, dust storms, smoke, snow storms etc. were defined as haze events. Fog events mostly occur during early morning, rarely lasting until 14:00 LT. Therefore, a day was defined as a foggy day if the occurrence of fog was recorded 2 Data and methodology at any time during the day. For each station the occurrences of low-visibility days, The North China Plain (NCP) is surrounded to the north by hazy days and foggy days during the period 1981–2010 the Yan Mountains, to the west by the Taihang Mountains were counted to analyse the spatial distribution of those and to the east by the Bohai Sea (Fig. 1). The western part three types of events. OMI Level 3 SO2 (OMSO2 of the NCP is affected by the warm and dry wind coming off Readme file v1.1.1, http://so2.gsfc.nasa.gov/Documentation/ the eastern Taihang Mountain slopes, which lead to increased OMSO2ReleaseDetails_v111_0303.htm) planetary bound- surface stability. Under the influence of the Bohai Sea, the ary layer column concentrations during 2005–2012 were east coast of the NCP often experiences moderate to strong used to show the correlation between the distribution of SO2 winds. The southern part of the NCP is rather flat and is an and the occurrence of haze days. important water vapour transport passageway. The spatial average annual and 10-year moving average The locations of the 64 meteorological observation sites frequency of occurrence during the period 1981–2010 were selected for this study are displayed in Fig. 1, with the names used to analyse the trend of the three types of events over of the sites given in Fig. 2a. All sites are located in the south- those 30 years. The trends are then compared to the SO2 ern part of the NCP, where the air pollution is most severe. emission trends inferred from the China statistical yearbooks The visibility, RH, wind speed and weather phenomenon ob- (National Bureau of Statistics of China, 1995–2010). The served at 14:00 local time (LT) during the period 1981–2010 spatial averages of monthly frequency of occurrence during

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Figure 2. (a) Average OMI SO2 PBL column amount during the period 2005–2012 with the location of the 64 stations and the number of (b) low visibility, (c) haze and (d) fog days during the period 1981–2010 in the southern NCP. the period 1981–2010 were applied to examine the long-term to the total average RH was compared against the decadal trend of those three types of events in different seasons. variation in annual haze days. The seasonal–decadal trend The average distribution and the linear trend of wind of the frequency of occurrence of haze-related RH val- speeds during the period 1981–2010 were respectively cal- ues falling in the range of RH < 50 %, 50 % < RH < 60 %, culated to reveal their impact on the distribution and trend 60 % < RH < 70 %, RH > 70 % was analysed to show the of the frequency of occurrence of the three types of events. impact RH has on haze in different seasons. NCEP final analysis meteorology data during 58 fog days in between January 2009 and February 2010 were used to reveal the characteristic wind field that has led to fog events in the southern NCP. Surface automatic weather station wind 3 Results and discussions data during May–December 2009 were used as supplements to analyse the distribution of the wind field in the entire NCP 3.1 The spatial distribution of low visibility, and how it impacts haze formation. haze and fog The average distribution of the number of days with 14 h RH > 70 % and the number of haze days with 14 h Low visibility, haze and fog events are formed through dif- RH > 70 % was calculated to show its relation to the dis- ferent mechanisms; hence their occurrences are determined tribution of low visibility, haze and fog events. To further by different parameters. This may lead to distinct spatial dis- study the influence of RH on haze events, the decadal vari- tributions of the frequency of occurrence. The spatial dis- ation of the ratio of the annual average haze related RH tributions of the occurrences of low visibility, haze and fog www.atmos-chem-phys.net/14/11949/2014/ Atmos. Chem. Phys., 14, 11949–11958, 2014 11952 G. Q. Fu et al.: The distribution and trends of fog and haze in the North China Plain

Table 1. The number (percentage) of stations under various count ranges of low visibility, haze and fog days during 1981–2010.

Days Stations ≥ 3000 2000–2999 1000–1999 500–999 100–499 < 100 Low visibility 2 (3 %) 6 (9 %) 22(34 %) 23 (36 %) 11 (17 %) 0 (0 %) Haze 0 (0 %) 2 (3 %) 12(19 %) 15 (23 %) 30 (47 %) 5 (8 %) Fog 0 (0 %) 0 (0 %) 8 (13 %) 47 (73 %) 9 (14 %) 0 (0 %) days during the period 1981–2010 in the southern NCP are 500–999 and 1000–1999 low-visibility days, while another depicted in Fig. 2b–d. 12 % of the stations have gone through more than 2000 low- Haze events are caused by either high aerosol loadings or visibility days and 17 % rather clean stations have only had the strong hygroscopic growth of aerosols. SO2 is the main 100–499 low-visibility days. Although haze events can be precursor of sulfate aerosols, which are highly hygroscopic very frequent, their impact is largely constrained within the and exist in abundance in the NCP (Liu et al., 2014). The av- limited area in the southwest; 22 % of the sites have expe- erage SO2 planetary boundary layer (PBL) column amount rienced more than 1000 haze days during the past 30 years, during the period 2005–2012 is depicted in Fig. 2a. High while 55 % of the sites have had fewer than 500 haze days. SO2 column concentrations are found on the western border, For fog events, however, 73 % of the sites have experienced which is caused on the one hand by the large amount of SO2 500–999 fog days during the past 30 years, suggesting that a emissions in this region and on the other hand by the spe- large area is under a similar influence of fog events. cial topography which leads to poor dispersion conditions. In all, low visibility, haze and fog days are distinctly dis- The high SO2 concentrations have led to large loadings of tributed in the NCP, because their formations are controlled highly hygroscopic sulfate aerosol, which can easily lead to by different mechanisms. The spatial distribution of haze haze events in this region. Figure 2c displays the distribution days is determined by the distribution of pollutant emissions frequency of haze days, which shows a very similar distri- and by the topography of the NCP, thus only influencing a bution to that of SO2 column concentration. Haze also most small area near the edge of the mountains. The distribution of frequently occurs on the western border of the NCP, along fog days is mainly determined by the topography and meteo- the eastern slope of the Taihang Mountains. rology, affecting a large area parallel to the mountains. Low The distribution of fog days is distinctly different than that visibility is mostly induced by haze events, thus showing a of haze days (Fig. 2d). Fog events most commonly occur similar distribution to that of haze days. along the centre line of the plain region, parallel to the southwest–northeast ridge of the Taihang Mountain. This suggests 3.2 The trends of low visibility, haze and fog during the that the formations of haze and fog are controlled by differ- period 1981–2010 ent physical processes. While the formation of haze mainly depends on the local aerosol pollution level, the formation The temporal trends of low visibility, haze and fog are in- of fog is not limited to the presence of cloud condensation fluenced by the variation of many factors such as pollutant nucleus under the regional pollution state of the NCP. The emissions, aerosol compositions and meteorological condi- special topography and meteorology in the NCP could be tions, etc. During the past 30 years, the NCP has undergone responsible for the spatial distribution of fog event occur- rapid economic developments accompanied by growing en- rences, which will be further discussed in Sect. 3.3.2. ergy consumptions. In the past decade, policies were made to Low-visibility events can be caused by both haze and fog. reduce pollutant emissions in the NCP in hope of improving The area with the most low-visibility days covers both the the air quality there. In this section, the decadal trends of low area with the most haze and fog days. Low visibility mainly visibility, haze and fog days will be examined to show the happens in the western part of the region, which is similar combined effect of various influencing factors. to the distribution of haze days. High centres were found around the three major cities Shijiazhuang, Xingtai and Han- 3.2.1 The decadal trends of low visibility, haze and fog dan. This indicates that low-visibility events in the NCP are mostly caused by haze, rather than by fog. The decadal variations of the annual and the 10-year moving Due to the distinct spatial distribution of low visibility, average of the occurrence of low visibility, haze and fog days haze and fog days, the spatial coverage of their influence during the period 1981–2010 are displayed in Fig. 3b. Before is also different. Table 1 lists the number (percentage) of 1995, due to the rapid economic development, increasing en- stations within various count ranges of low visibility, haze ergy consumption led to increasing numbers of haze and fog and fog days during the period 1981–2010. In the southern days, which resulted in more frequent low-visibility events. NCP, 34 and 36 % of the stations have respectively observed During the period 1995–2003, with the development and employment of waste gas processing techniques, occurrences of

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2001–2010, with slopes of 10 days 10 year−1. The number of low-visibility days increased before the period of 1996–2005, with a maximum slope of 17 days 10 year−1 during the period 1983–1992. Afterwards, low-visibility events were found to be less frequent; the maximum decreasing slope was found during the period 2001–2010, reaching 16 days 10 year−1. Generally, the frequencies of occurrence of low visibility, haze and fog in the NCP have all increased during the period 1980–1990. Due to the effect of SO2 emission reduction during the past few years, the occurrences of low visibility, haze and fog have all decreased back to a similar level as in 1980.

3.2.2 The seasonal–decadal trends of low visibility, haze and fog

Low visibility, haze and fog are strongly influenced by meteorological conditions, which in the NCP vary distinctly with season. This may cause different decadal variation in different seasons. Figure 4 shows the seasonal–decadal trends of the frequency of occurrence of low visibility, haze and fog during the period 1981–2010. During the past 30 years, haze events were most common during the heating season (Nov–Mar) and least common during the end of spring and early autumn (Fig. 4d). After 1990, haze events started to appear not only during the heating season but also during summertime (Fig. 4b2). From the 10-year moving average frequency of occurrence of haze Figure 3. (a) The annual SO2 emissions in Hebei Province inferred in spring, summer, autumn and winter (Fig. 4b1), it can be from the China statistical yearbooks (National Bureau of Statistics seen that summertime haze occurrence frequency has been of China, 1995–2010); (b) the annual (solid lines) and the 10-year continuously increasing in the past 3 decades, while that in moving average (dashed lines) occurrence of low visibility, haze and fog days among the 64 stations during the period 1981–2010; wintertime has been declining since 1995. In the past, resi- (b) the slope of the linear fits to the 10-year moving averages of low dential and commercial heating during winter has led to high visibility, haze and fog days among the 64 stations during the period emissions of soot, which were responsible for the degrada- 1981–2010. tion of visibility. With the development of central heating and waste gas processing techniques, emissions from heating processes have been gradually declining, leading to fewer low visibility, haze and fog days entered a steady stage. After haze events during winter. However, atmospheric pollution 2003, in preparation for the 2008 Beijing Olympic Games, has become more complex during recent years. During sum- under the influence of emission control policies, the number mer, the atmosphere becomes highly oxidative, and aerosol of low visibility, haze and fog days drastically dropped. The pollution coexists with high concentrations of O3 and volatile trend of low visibility and haze days agrees well with that organic compounds (VOCs) (Ran et al., 2011), which con- tribute to the fast aging and secondary formation of aerosols. of the SO2 emissions (Fig. 3a), suggesting that the control in emissions during the last decade effectively led to increased Aged aerosols are more hygroscopic and can easily lead to visibility and the decline in the number of hazy days. haze events under suitable RH conditions (Chen et al., 2012). The slopes of the linear fits to the 10-year moving aver- The increased use of air conditioning during summertime ages of low visibility, haze and fog occurrence days during has resulted in increasing energy consumption trends (Zhang, the period 1981–2010 are shown Fig. 3c. The maximum in- 2004), which could also lead to increased aerosol loading and creasing slope for haze events can be found between 1989 reduced visibility during summer. and 1998, reaching 17 days 10 year−1. After the period of Fog events appear most frequently during winter and least 1994–2003, haze events generally declined, reaching a max- frequently during spring until early summer. The frequency imum decreasing slope of 11 days 10 year−1 during the pe- of occurrence of fog during the past 30 years shows no sig- riod 1996–2005. After the period of 1987–1996, fog occur- nificant trend in spring and summer, but shows a declining rences show a variable but generally decreasing trend. Ma- trend during winter after 1990 (Fig. 4c1). jor decreases occurred during the periods of 1990–1999 and www.atmos-chem-phys.net/14/11949/2014/ Atmos. Chem. Phys., 14, 11949–11958, 2014 11954 G. Q. Fu et al.: The distribution and trends of fog and haze in the North China Plain

Figure 4. (1) The trend of the 10-year moving average frequency of occurrence of (a) low-visibility events at 14:00 LT, (b) haze events at 14:00 LT and (c) fog events during the day among the 64 stations during spring (MAM), summer (JJA), autumn (SON) and winter (DJF). (2) The seasonal–annual variation of the average frequency of occurrence of (a) low-visibility events at 14:00 LT, (b) haze events at 14:00 LT and (c) fog events during the day among the 64 stations and (d) the seasonal variation of the average frequency of occurrence of low-visibility events at 14:00 LT, haze events at 14:00 LT and fog events among the 64 stations during 1981–2010.

Low-visibility events are caused by both haze and fog. the Taihang Mountains, the winds all come from the south- Their combined effect has resulted in high frequency of oc- eastern direction, which can transport the emitted pollutants currence of low visibility during winter and lower ones dur- from the eastern part of the plain area to the western part. ing spring and summer (Fig. 4a1). Low-visibility events dur- The weak north-western winds in the mountains block pollu- ing summertime have been continuously increasing in the tant transport, leading to the accumulation of heavy loadings past 3 decades, while those during spring, autumn and winter of aerosol and its precursors along the foot of the Taihang have been decreasing since 2000. Mountains. This explains why haze events are most common In all, wintertime low visibility, haze and fog days have in this region. been declining, while summertime low visibility and haze Figure 6a displays the distribution of the average 14 h days have been increasing due to the variation in aerosol (LT) wind speed during the period 1981–2010. Average wind composition and the summertime high RH that favours the speeds at 14 h (LT) decrease from the southeast to the north- hygroscopic growth of particles. Additionally, the increased west. Lowest values are found in the northwest corner, de- energy consumption due to air conditioning might also lead creasing below 3 m s−1, while similar values were also de- to increased summertime aerosol loadings and decreased vis- tected in Xingtai and Handan, two heavily polluted cities ibilities. in the southern corner. Average wind speeds near the coast (in the vicinity of Cangzhou) and in the southeast corner are 3.3 The impact of wind on low visibility, haze and fog large, reaching over 5 m s−1. The low wind speeds are caused primarily by the Taihang Mountains, which slow down west- 3.3.1 The impact of wind on haze erly winds before they reach the plain region. As can be seen from Fig. 5, the Taihang Mountains are higher in the north Wind direction can influence the transport of pollutants, thus and lower in the south, which explains why wind speeds are determining the spatial distribution of visibility degrading lowest in the northwest corner. Another factor influencing pollutants. Wind speed greatly influences the atmospheric the surface wind speed is the surface roughness. Large cities stability and the atmospheric dispersion capability. Low wind with densely distributed high buildings will add to the sur- speeds suggest that the atmosphere is rather stable and the face roughness and slow down near surface winds. dispersion of local emissions is limited. Although dispersion abilities are weakest in the northwest Figure 5a shows the spatial distribution of the average 14 h corner, due to relatively lower pollutant emissions in the (LT) surface wind direction during May–December 2009 in mountain areas, low visibility and haze events are rare on the the entire NCP. The Yan and Taihang mountains are gov- northwestern edge. However, the area near Baoding, where erned by northwest winds, while the plain area is dominated the pollution level is slightly higher than in the mountain by winds from the south. Along the south-western edge of

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Figure 5. (a) Averaged 14 h surface wind direction during May–December 2009 given by 307 AWS stations. (b) The orographic geopotential height above sea level (shading), the average 8 h 1000 hPa NCEP ﬁnal analysis wind ﬁeld (black arrows) and the orographically generated wind convergence line (red line) of 58 fog days during January 2009–February 2010.

Figure 6. (a) Distribution of the average 14 h (LT) wind speed during 1981–2010 in the southern NCP; (b) distribution of the linear slope of the annual wind speed during 1981–2010 in the southern NCP; (c) variation (dashed lines) and linear trend (solid lines) of the regional average 14 h (LT) wind speed and that of Cangzhou and Shijiazhuang during 1981–2010. areas and distinctively lower than over the polluted region decrease in wind speed suggests that the atmosphere has be- in the southwest, is heavily affected by the low wind speeds, come more stable and dispersion capabilities have weakened showing frequently occurring low visibility and haze events. throughout the entire region, which calls for even more ex- The southwestern edge of the southern NCP region suffers treme efforts to control emissions in order to prevent haze from high pollutant emissions and is subject to conditions events. that lead to relatively weak dispersion. The low average wind speed centres in Shijiazhuang, Xingtai and Handan (Fig. 7a) 3.3.2 The impact of wind on fog conform to the high haze day count centres (Fig. 2c). Figure 6b shows how the linear slope of the annual wind The formation of fog needs a supersaturated water vapour en- speed during 1981–2010 is distributed in the southern NCP. vironment, which requires favourable meteorological condi- Over the entire region, wind speeds have been decreasing tions. Figure 5b shows the average 08:00 (LT) 1000 hPa wind in the past 30 years. Large decreasing slopes were found ﬁeld during 58 fog days that occurred between January 2009 in those regions with high average wind speeds, but also in and February 2010, on which more than 10 stations reported parts of the northeast corner and in Shijiazhuang, which is the occurrence of fog. Due to the topography of the NCP, an the capital of Hebei Province. The regional average 10-year orographically generated, boundary layer wind convergence decreasing slope reaches 0.2 m s−1 10 year−1, while that in −1 −1 line going from southwest to the northeast is formed as in- Shijiazhuang reaches over 0.2 m s 10 year (Fig. 6c). The dicated by the red line in Fig. 5b. The convergence line is www.atmos-chem-phys.net/14/11949/2014/ Atmos. Chem. Phys., 14, 11949–11958, 2014 11956 G. Q. Fu et al.: The distribution and trends of fog and haze in the North China Plain

Figure 7. (1) The decadal trend of the 10-year moving average frequency of occurrence of 14h (LT) haze event RH in the range of (a) RH < 50 %, (b) 50 % < RH < 60 %, (c) 60 % < RH < 70 % and (d) RH > 70 % during spring (MAM), summer (JJA), autumn (SON) and winter (DJF). (2) The seasonal–decadal variation of the frequency of occurrence of 14 h (LT) haze event RH in the range of (a) RH < 50 %, (b) 50 % < RH < 60 %, (c) 60 % < RH < 70 % and (d) 70 %< RH <80 % and (e) the seasonal variation of the average frequency of occurrence of 14h (LT) haze event RH in the range of (a) RH < 50 %, (b) 50 % < RH < 60 %, (c) 60 % < RH < 70 % and (d) RH > 70 % during the period 1981–2010. also parallel to the ridge of the Taihang Mountains, coincid- with 14 h (LT) RH > 70 % (Fig. 8b) is similar to that of ing with the zone with the most frequent fog events (Fig. 2d). the number of all days with 14 h (LT) RH > 70 % (Fig. 8a), Fog events usually occur during the night, when the moun- only with signiﬁcantly smaller numbers to the east of Shiji- tain areas cool off faster than the plain area, forming a tem- azhuang, because haze is not as severe in that region. Com- perature gradient. To the west of the convergence line, near pared with the distribution of the total number of low visibil- surface winds are dominated by cold north-westerly moun- ity and haze days (Fig. 2b–c), it can be noted that the frequent tain winds, while to the east of the convergence line, surface low-visibility events along the southern edge of the Taihang winds are north-easterly or south-easterly. With the north- mountain were caused primarily by the heavy aerosol pol- easterly winds comes the warm humid air from the Bohai lution and not by haze events associated with high RH. The Sea, while the south-easterly path is the most typical water low-visibility events in the vicinity of Shijiazhuang, however, vapour transport passage way for the entire NCP region. The were not only caused by severe pollution but were also asso- convergence zone will hence favour the accumulation and ciated with high RH events, indicating that the hygroscopic convergence of water vapour, and lead to the formation of growth of aerosols plays an important role in the visibility fog in this area. impairment in this region. Although aerosol pollution is not as severe in the south-eastern part of our area of study, a large 3.4 The impact of RH on low visibility, haze and fog fraction of haze days are associated with high RH events, suggesting that the high RH in this region is able to impair The ambient RH can inﬂuence the visibility by affecting the visibility even if the aerosol concentration is not very high. hygroscopic growth and scattering abilities of atmospheric The hygroscopic growth induced light scattering of aerosols < aerosols. Chen et al. (2012) suggest that, under RH 80 %, plays a dominant role in the degradation of visibility in this visibility is highly dependent on dry aerosol volume con- region. centrations. The hygroscopic growth of aerosols in this RH The 14 h (LT) RH values that were accompanied by haze range only becomes important for visibility impairment if the events (haze event RH) were sorted out and its frequency of aerosol loading is high, while for RH greater than 80 % the occurrence in the range of RH < 50 %, 50 % < RH < 60 %, hygroscopic growth of aerosols can greatly affect visibility, 60 % < RH < 70 % and RH > 70 % was calculated for each even under average aerosol pollution levels. station in each month during the period 1981–2010. Fig- The distribution of the number of days with 14 h (LT) ure 7a2–d2 show the regional average seasonal–decadal vari- > RH 70 % and the number of haze days with 14 h (LT) ation of the frequency of occurrence of haze event RHs. It > RH 70 % are depicted in Fig. 8. High RH days most fre- can be noted that haze events mostly occur under RH > 60 % quently occur in the southeast, where there is a water vapour during summer and early autumn, while during late au- transport passageway, and along the convergence line, which tumn, winter and spring, haze events mostly occur under was observed to be favourable for the formation of fog events RH < 60 %. This suggests that haze events during the warm in Sect. 3.3.2. The distribution of the number of haze days

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Figure 8. The distribution of (a) the number of days with 14 h (LT) RH above 70 % and (b) the number of haze days with 14 h (LT) RH above 70 % during 1981–2010 in the southern NCP.

show increasing trends during the past 30 years. A possible cause could be the fact that atmospheric pollution has become more complicated over the years, leading to a higher fraction of secondary aerosols, which are more hygroscopic and are more likely to impair visibility through hygroscopic growth processes. To further study if this phenomenon exists throughout the entire region, the decadal variation of haze event RH, annual average RH and annual haze days at four representative stations were analysed as is depicted in Fig. 9. The RH associated with haze events is typically higher than the average RH values. Zanhuang station shows continuously increasing number of haze days throughout the past 30 years, while Hengshui displays a continuous decline throughout the past 20 years (Fig. 9b). Shijiazhuang and Longyao have both undergone an increase before 2000 and a decrease thereafter. Signiﬁcant increasing trends can be found in the ratio of haze event RH and average RH at all four stations (Fig. 9a). This means that a higher fraction of the haze events are now Figure 9. (a) The decadal variation of the haze event associated RH caused by the hygroscopic growth of aerosols. The reduction divided by the average RH and (b) the annual number of haze days in primary aerosol emissions further ampliﬁes this effect. during the period 1981–2010 at Shijiazhuang, Hengshui, Longyao and Zanhuang. 4 Summary

In this study, the spatial distribution and decadal variation of season are caused both by high aerosol loadings and their hy- low visibility, fog and haze events in the most polluted south- groscopic growth, while those in the cold seasons are mostly ern part of the NCP during the past 30 years were analysed induced by high aerosol loadings. and the impact of wind and RH on those events was investi- The frequency of occurrence of haze events under low gated. RH (RH < 50 %) in autumn, winter and spring has decreased Haze and fog are distinctly distributed, which was deter- over the past 30 years, with a slight rebound in the winter mined by the topography of the NCP and the distribution of time data at the end of the last decade. This indicates that wind speed and wind direction. Haze occurs mostly along the aerosol loadings during winter have declined, which could south-western edge of the plain region, while fog mostly oc- be the result of effective emission control measures during curs within the central band area parallel to the ridge of the heating seasons. The frequency of occurrence of haze events Taihang Mountains. under higher RH (RH > 60 %) in autumn, winter and spring www.atmos-chem-phys.net/14/11949/2014/ Atmos. Chem. Phys., 14, 11949–11958, 2014 11958 G. Q. Fu et al.: The distribution and trends of fog and haze in the North China Plain

Annual low visibility, haze and fog days have shown in- visibilities for hazy days in the North China Plain, Atmos. Chem. creasing trends before 1995, have entered a steady stage dur- Phys., 12, 4935–4950, doi:10.5194/acp-12-4935-2012, 2012. ing the period 1995–2003 and have drastically dropped there- China Statistical Yearbook: 1995–2010, http://www.stats.gov.cn/ after during the preparation stage for the Beijing Olympic tjsj/ndsj/, access: 26 March 2014. Games in 2008. Low visibility, haze and fog events all oc- Danielson, J. J. and Gesch, D. B.: Global multi-resolution terrain el- curred most frequently during the heating season in the past evation data 2010 (GMTED2010): US Geological Survey Open- File Report, 26, 2011–1073, 2011. 3 decades. Wintertime haze and fog both show decreasing Deng, J., Du, K., Wang, K., Yuan, C.-S., and Zhao, J.: Long-term trends, as a benefit of the improvements in central heating atmospheric visibility trend in Southeast China, 1973–2010, At- and desulfurization techniques. Summertime haze, however, mos. Environ., 59, 11–21, 2012. displayed continuously increasing trends during the period Doyle, M. and Dorling, S.: Visibility trends in the UK 1950–1997, 1981–2010. Atmos. Environ., 36, 3161–3172, 2002. Both the distribution of the wind field and the decadal vari- Fu, C. and Wu, J.: The Different Characteristics of Sunny Visibility ation of wind speeds have great impacts on the occurrence over Southwest China in Recent 50 Years, Proc. Environ. Sci., of low visibility, haze and fog events. South-easterly winds Part A, 10, 247–254, 2011. in the southern part of the NCP which are blocked by the Liu, H. J., Zhao, C. S., Nekat, B., Ma, N., Wiedensohler, A., weak north-western winds in the Taihang Mountains has re- van Pinxteren, D., Spindler, G., Müller, K., and Herrmann, H.: sulted in high pollutant concentrations along the foot of the Aerosol hygroscopicity derived from size-segregated chemical composition and its parameterization in the North China Plain, mountain, which has led to frequent haze events. The con- Atmos. Chem. Phys., 14, 2525–2539, doi:10.5194/acp-14-2525- vergence zone parallel to the ridge of the Taihang Mountains 2014, 2014. in the central area of the southern NCP was responsible for Ma, N., Zhao, C. S., Nowak, A., Müller, T., Pfeifer, S., Cheng, Y. the frequent fog events in this region. Wind speed has been F., Deng, Z. Z., Liu, P. F., Xu, W. Y., Ran, L., Yan, P., Göbel, decreasing throughout the entire southern NCP, resulting in T., Hallbauer, E., Mildenberger, K., Henning, S., Yu, J., Chen, more stable atmospheric conditions and thus stronger inver- L. L., Zhou, X. J., Stratmann, F., and Wiedensohler, A.: Aerosol sions and reduced dispersion capability, which calls for even optical properties in the North China Plain during HaChi cam- increased efforts to control emissions in order to prevent haze paign: an in-situ optical closure study, Atmos. Chem. Phys., 11, events. 5959–5973, doi:10.5194/acp-11-5959-2011, 2011. Haze and low-visibility events were strongly influenced Molnár, A., Mészáros, E., Imre, K., and Rüll, A.: Trends in visibil- by the ambient RH. The frequent high RH events near Shi- ity over Hungary between 1996 and 2002, Atmos. Environ., 42, 2621–2629, 2008. jiazhuang and in the south-eastern part of the region favours Ran, L., Zhao, C. S., Xu, W. Y., Lu, X. Q., Han, M., Lin, W. L., the hygroscopic growth of aerosols and plays a dominant role Yan, P., Xu, X. B., Deng, Z. Z., Ma, N., Liu, P. F., Yu, J., Liang, in the visibility impairment in those areas. Under RH below W. D., and Chen, L. 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Wu, J., Fu, C., Zhang, L., and Tang, J.: Trends of visibility on sunny days in China in the recent 50 years, Atmos. Environ., 55, Acknowledgements. This work is supported by the National 973 339–346, 2012. project of China (2011CB403402), the National Natural Science Xu, W. Y., Zhao, C. S., Ran, L., Deng, Z. Z., Liu, P. F., Ma, N., Foundation of China under grant no. 41375134, the Beijing Natural Lin, W. L., Xu, X. B., Yan, P., He, X., Yu, J., Liang, W. D., and Science Foundation (8131003), the Beijing Municipal Science and Chen, L. L.: Characteristics of pollutants and their correlation to Technology Plan Project no. Z131100006113013. meteorological conditions at a suburban site in the North China Plain, Atmos. Chem. Phys., 11, 4353–4369, doi:10.5194/acp-11- Edited by: D. Covert 4353-2011, 2011. Zhang, Q.: Residential energy consumption in China and its comparison with Japan, Canada, and USA, Energ. Build., 36, References 1217–1225, h 2004. 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